This application claims priority to an application entitled xe2x80x9cPlanar Inverted F Antennaxe2x80x9d filed in the Japanese Patent Office on Apr. 17, 2001 and assigned Ser. No. 2001-118186, the contents of which are herein incorporated by reference.
1. Field of the Invention
The present invention relates to a planar inverted F antenna (PIFA) having stable radiation characteristics for use in a portable terminal.
2. Description of the Related Art
Mono-pole antennas and planar inverted F antennas have been used for small size devices such as portable terminals.
FIG. 5 is a perspective view of a conventional planar inverted F antenna. Referring to FIG. 5, a planar insulator 1 has a radiation device 2 on a portion of one surface thereof and a grounding plate 3 as wide as the radiation device 2 on the other surface thereof. The radiation device 2 is connected to the grounding plate 3 via a conductor 8. A central conductor 5 of a coaxial cable 4 is electrically connected to the radiation device 2 and an outer conductor 6 of the coaxial cable 4 is electrically connected to the grounding plate 3.
The radiation characteristics of the planar inverted F antenna shown in FIG. 5 depend on the size and shape of the grounding plate 3. Therefore, the size of the grounding plate 3 is set or antenna characteristics are adjusted according to the grounding plate 3.
If the above planar inverted F antenna is to be disposed in a small space, the grounding plate 3 needs to be made smaller. However, scaling-down the size of the grounding plate 3 causes impedance mismatch and increases the vulnerability to electrical interference from adjacent circuits or metal components. As a result, the radiation characteristics of the antenna are deteriorated or the antenna operates at an incorrect resonant frequency.
In FIG. 5, if the grounding plate 3 becomes too small, a leakage current, which is not observed with a sufficiently large grounding plate, flows through the outer conductor 6 of the coaxial cable 4. In this state the radiation device 2 and the grounding plate 3 exhibit characteristics of a dipole antenna, that is, the miniaturized groundling plate 3 is virtually connected to the coaxial cable 4 and the radiation device 2 not by an unbalanced feed line but by a balanced feed line. The leakage current flowing through the coaxial cable 4 deteriorates the antenna""s characteristics and renders the antenna vulnerable to ambient influences.
If an antenna is attached to a side of the cover of a laptop computer that co-functions as a display, its ambient environment has different influences over the antenna in a closed state (a standby state) and in an open state. When antenna characteristics are adjusted in one of the states, the antenna is influenced by nearby objects in the other state. Thus, leakage current changes and impedance mismatch is generated. As a result, the resonant frequency of the antenna is changed or its radiation characteristics deteriorate.
It is, therefore, an object of the present invention to provide a planar inverted F antenna which is not vulnerable to ambient influence although the antenna is made smaller.
It is another object of the present invention to provide a planar inverted F antenna that exhibits stable radiation characteristics.
The above and other objects of the present invention are achieved by providing a planar inverted F antenna with stable radiation characteristics, which is not vulnerable to ambient influence. In the planar inverted F antenna, an insulator has a radiation device formed on one surface and a grounding plate formed on the other surface thereof. A coaxial cable has a central conductor electrically connected to the radiation device and an outer conductor electrically connected to the grounding plate at two points spaced from each other by approximately a quarter wavelength of current flowing through the outer conductor. If leakage current flows along the outer conductor, the leakage current is negated by an inverse-phase current flowing through the groundling plate.
It is preferred that the groundling plate has a length of about a quarter wavelength of the current flowing through the outer conductor.
It is preferred that the insulator is formed of a dielectric material having a high dielectric constant.